The present invention relates generally to a cartridge filter and, more particularly, to a cylindrical cartridge filter having a pleated filter element made of a non-woven fabric that can be traversed by a stream of air in the radial direction. The filter is generally used in situations calling for initial pressure losses of less than 300 Pa at an air flow 250 m.sup.3 /m.sup.2 h.
In known filters of this type, the cartridge contains a filter medium that is made of one or more layers, each having a homogeneous structure, wherein the upstream layer in regard to the air flow is generally very dense and has extremely small pores. Due to the pleating and the thinness of the medium, the filter has a comparatively large surface area, so that the measured pressure drop of the filter cartridge is very low. It is quite common for such cartridge filters to contain as much as 15 to 20 m.sup.2 of filter media. Such a cartridge filter is generally only useful for extracting dust, where high concentrations (greater than 1 mg/m.sup.3) of a relatively coarse dust (larger than 1 micrometer) are present, such as in industrial dust control applications. The dust particles that are separated out of the air in the course of purification during normal use collect as a removable filter cake on the dense upstream (incident flow) side of the cartridge filter.
Since, as a rule, the dust concentration in the fresh air filtration is less than 1 mg/m.sup.3, and the air contains large concentrations of fine dust particles (smaller than 0.5 microns), the extremely small pores of the filter medium become quickly clogged by depth filter effects and a removable dust cake can not build up on the surface of the medium. The net effect is to make it impossible to effectively clean the filter by normal reverse air flow or pulse jet cleaning. This leads to a rapid rise in pressure loss after a short period of use.
It is also known, with respect to air filtration, to use so-called "deep-bed" filters. These filters comprise soft non-woven fabric mats of a thickness of 8 to 100 mm, whose porous structure becomes continuously denser in the direction of the clean (downstream) air side. As a result, the dust particles to be separated out of the air to be purified are able to penetrate to varying depths dependent on particle size, into the inside of the filter mats, until they are held fast therein. The pressure drop that ensues during normal operational use is nearly constant over long periods of time. However, because of their considerable voluminousness and softness, filter mats of this type can not be pleated. Hence, filter mats required for a given volume of air, have a comparatively large upstream surface area, which makes it necessary to provide especially large aggregates for their installation. The reverse pulse jet cleaning of such filters is not possible. For this reason, depth filters are primarily used in the air supply filtration of air conditioning systems (HVAC).
German Patent 4304036 discuses the construction of a cartridge filter. In particular, it shows a pleated non-woven fabric that is transformed into a cylindrical shape and is fixed in a holding device. The non-woven fabric used in manufacturing the known cartridge filter had always been structured homogeneously over its entire cross-section. Customary surface-filter media have a porosity of less than 85%. A non-woven fabric having a porosity greater than 90% and a thickness greater than 1.5 mm, either cannot be pleated at all or only with considerable difficulty. For the most part, during normal operational use, it is too soft to oppose the oncoming flow of air with the necessary static resistance. The resultant plastic deformations cause the folds to rest on one another and the pressure losses to suddenly rise. One can counteract this deformation by backing the pleated media with reinforcement materials (wire mesh fabric or non-woven carriers), but thisentails considerable expense.
It is known to make a laminate composite filter from several discrete, separate layers of non-woven fabrics. Such an approach is set forth in U.S. Pat. No. 5,282,106 to Seiler et al. However, this approach requires that the discrete layers be joined to form the composite, and the joining process reduces the effective surface area of the filter, thereby compromising the basic efficacy of the filter for a given size.
The present invention is directed to the development of a pleated cartridge filter that affords a pressure drop that is essentially unchanging over long periods of use and which is suited for an application in the area of fresh air filtration.